Generally, an oxide superconducting wire rod, represented by YBa2Cu3O7-x, has high current transport capacity and excellent critical current characteristics in high magnetic fields. Therefore, it is expected that such an oxide superconducting wire rod will enable large-capacity power equipment to have a small size, high efficiency and large capacity when it is applied to power cables, industrial motors, generators, and the like in the future. The oxide superconducting wire rod, as shown in FIG. 1, includes a metal substrate A, a buffer layer B and a superconducting layer C, and the current transport characteristics thereof change greatly depending on the orientation of superconductor grains. Therefore, in order to manufacture a superconducting wire rod having a high critical current density (Jc), superconductor grains must be highly biaxially oriented. In particular, a buffer layer, serving to transmit the orientation of a metal substrate to a superconducting layer and to suppress the reaction between a metal substrate and a superconducting layer, plays an important role in the manufacture of a superconducting wire rod, and the cost of preparing the buffer layer accounts for most of the total cost of manufacturing the superconducting wire rod. Therefore, in order to impart functionality to the buffer layer, the buffer layer must be grown on the metal substrate while being biaxially oriented thereon, and, in order to suppress the reaction between the metal substrate and the superconducting layer, the buffer layer must have high density or be thick. Further, the properties of the metal substrate must not be deteriorated during the process of forming the buffer layer on the metal substrate. In order to accomplish the above object, various materials, such as CeO2, YSZ, Y2O3, RE2Zr2O7 (RE=La, Sm, Ce, etc.), SrTiO3, and the like, are currently used to form the buffer layer.
Currently, in order to form a buffer layer on a metal substrate produced through a process of manufacturing an oxide superconducting wire rod, high-vacuum methods, such as sputtering, pulsed laser deposition (PLD), thermal evaporation, metal organic chemical vapor deposition (MOCVD), etc., or wet chemical methods, such as metal organic deposition (MOD), etc., are used.
The high-vacuum methods are disadvantageous in that, since they must be performed in a high vacuum, that is, at a low pressure of 10˜5 Pa, expensive high-vacuum systems and advanced high-vacuum technologies are required, thus decreasing process stability and economic efficiency, which are prerequisites to the practical use of the superconducting wire rod. In contrast, the wet chemical methods, such as metal organic deposition (MOD), etc., are economically advantageous in that they need not be performed in a high vacuum, and include simple processes, for example, coating and heat-treatment.
However, when a buffer layer is formed through metal organic deposition (MOD), a highly oriented and densified buffer layer can be formed only when heat-treatment is conducted at 1000° C. or higher, generally at 1100° C. Therefore, the wet chemical methods are also problematic in that the crystal grain boundaries of a metal substrate are transformed, and thus grooves are formed in the surface thereof. The grooves, formed along the crystal grain boundaries of the metal substrate, worsen the connection between superconducting crystal grains, thus deteriorating the critical current characteristics of a superconducting layer, at the time of forming a superconducting layer. In particular, the grooves become an obstacle to a process of extending a superconducting wire rod, which is necessary for practical application of the superconducting wire rod.
When such problems, occurring in the process of forming a buffer layer, are overcome, and simultaneously, a highly oriented and densified superconducting layer is formed at low cost, the productivity of an oxide superconducting wire rod is maximized, thus having far-reaching effects on the practical use of the oxide superconducting wire rod.
Therefore, in order to overcome the above problems, methods of forming a buffer layer, which can form a highly oriented buffer layer even when it is heat-treated at a low temperature of 1000° C. or lower, are required.